ADC- essential for planetary?

[johnturley]

42 minutes ago, John said:

On a practical note, how much inwards focuser travel do the ZWO and Altair ADC's eat up ?

I've seen a figure of 57mm for the ZWO but that sounds a lot to me

Thanks

You can reduce this to about 45 mm by replacing the T thread to 1.25 in adaptor on the ZWO ADC by a T thread to 2 in adaptor, or fitting a Tele Vue 'In Travel' adaptor, as I have done on mine, assuming of course you have a 2 in focuser.

John 

[John]

Thanks both.

The scopes that I would use it with are F/9 and F/9.2 refractors. A flourite doublet and a triplet. The ED120 is F/7.5 but inward focus will be an issue with that one - I only have around 30mm when at focus. The F/9 and F/9.2 are binoviewer friendly (not that I use one) without a barlow so I am hoping that I can just pop the ADC on and use it with those without a barlow or other adaptation ?

 

 

 

 

[BGazing]

9 hours ago, John said:

On a practical note, how much inwards focuser travel do the ZWO and Altair ADC's eat up ?

I've seen a figure of 57mm for the ZWO but that sounds a lot to me

Thanks

I did not measure, but then again when I use it in mono I use it with PM2.5 and I do not have to remove extension on Tak.

Just use some barlow or (ideally) PM2.5 with t2 connection to ADC...if you happen to have one.

Below f/10 you will get astigmatism. I would bump up even those f/9 scopes (DL?) although it just might be enough.

 

[Rob]

Good thread here, and I've been following from the start. For me it just feels like I'd be tweaking kit constantly rather than observing!. The atmosphere is constantly moving, same as the 'seeing'. We all know those moments where seeing suddenly settles and you have that Wow moment.

I'm sure for some this maybe a great tool. But I have found over the last few years that a small ED does seem to offer benefit while the planets are low.

[Rusted]

Any thoughts on the average, refractor objective being closer to the ground with regards to seeing conditions for low objects?
Rather than there being a hugely greater, linear depth of atmosphere between you and the [low] external object.

I routinely get clearest and steadiest seeing conditions for solar imaging when the sun is lowest in the sky.
This would strongly suggest that I need a dispersion corrector far more than occasional, planetary observers and imagers.

Fortunately, narrowband H-alpha is [probably] relatively immune to atmospheric dispersion compared with broad spectrum "white light."
Though it could well be argued that atmospheric dispersion could well cause off-band effects within the very narrow H-alpha spectrum.

Why aren't these correctors fitted with a small pendulum to ensure their correct orientation to the horizon?
That would save messing about with a torch and a bubble level while your eyes are supposed to be dark adapted.

 

[fate187]

11 hours ago, markse68 said:

well if the scope is faster than f10 then it’s a good idea as the prisms can introduce other aberrations apparently. otherwise not but it could help with in- focus travel too? I’m using a 2x with my f8. gives my orthos a bit more er too which is nice. The ultra-ultra-expensive Gutekunst model (they do 2 versions)  has a built in barlow

The more expensive version of the Gutekunst is offered with a barlow and has a larger diameter (38 vs 28mm), to offer a better large field experience. For planetary and even 1.25" eyepieces I don't see vignetting.

10 hours ago, John said:

Thanks both.

The scopes that I would use it with are F/9 and F/9.2 refractors. A flourite doublet and a triplet. The ED120 is F/7.5 but inward focus will be an issue with that one - I only have around 30mm when at focus. The F/9 and F/9.2 are binoviewer friendly (not that I use one) without a barlow so I am hoping that I can just pop the ADC on and use it with those without a barlow or other adaptation ?

 

 

 

 

As I have written here, I didn't barlow the ZWO ADC when I had it. This was because I didn't know. With the Gutekunst ADC Mr. Gutekunst also recommends barlowing for f<10. However, I have not seen a better performance with my scopes using them at their native f/6.8 to f/9. So experience may vary. I think Gutekunst is also recommending this for the last tiny bit super optimal performance. As BGazing writes using a barlow on the ZWO will reduce astigmatism. However, Gutekunst recommends the barlow to reduce chromic abberations by the prisms. No native astigmatism here.
BTW John, I can use the LZOS 130 f/9 (I think you have the same tube version with 2.5" focuser from FT?) and I can reach focus with ADC/diagonal/GPC1.25/bino combination. No barlowing needed when I use the GPC, because this adds enough back focus. Granted, the Gutekunst eats up 48mm optical path, compared to the smaller ZWO unit as pictured above ~ 30mm giving a little more room for focusing.

49 minutes ago, Rusted said:

Any thoughts on the average, refractor objective being closer to the ground with regards to seeing conditions for low objects?
Rather than there being a hugely greater, linear depth of atmosphere between you and the [low] external object.

I routinely get clearest and steadiest seeing conditions for solar imaging when the sun is lowest in the sky.
This would strongly suggest that I need a dispersion corrector far more than occasional, planetary observers and imagers.

Fortunately, narrowband H-alpha is [probably] relatively immune to atmospheric dispersion compared with broad spectrum "white light."
Though it could well be argued that atmospheric dispersion could well cause off-band effects within the very narrow H-alpha spectrum.

Why aren't these correctors fitted with a small pendulum to ensure their correct orientation to the horizon?
That would save messing about with a torch and a bubble level while your eyes are supposed to be dark adapted.

 

Imho the bubble level is not necessary and rough tweaking of the orientation is enough. It gets more complicated in mirror systems like newtons though. There neither bubble level nor pendulum will work.

Edited July 26, 2020 by fate187

[John]

1 hour ago, BGazing said:

I did not measure, but then again when I use it in mono I use it with PM2.5 and I do not have to remove extension on Tak.

Just use some barlow or (ideally) PM2.5 with t2 connection to ADC...if you happen to have one.

Below f/10 you will get astigmatism. I would bump up even those f/9 scopes (DL?) although it just might be enough.

 

Thanks for that.

I don't want to start having to use a barlow or powermate with the scopes - yet more glass to add to the light path.

 

[Paz]

Here's a bit of feedback to add from observing Jupiter tonight  at about 14.5 degrees altitude with a C8+Baader prism diagonal+ADC+17.3mm Delos in quite poor seeing.

Best setting seemed to be as per the picture below just  using "the force" to set it up (with the eyepiece pointing up vertically and the ADC adjusting levers sticking out to the left).

It clearly cleans up atmospheric dispersion but the improvement in detail observable was very marginal on this occasion.

Edited July 27, 2020 by Paz

[johninderby]

Couple of mods to the Altair ADC.

Replaced the supplied eyepiece holder with a TS one as the original one took a lot of force to insert an eyepiece. The TS is the same height.

Wanted to be able to rotate the ADC so fitted a short eyepiece holder to the diagonal and shortened a T2 nosepiece in the lathe to suit.

Found the black adjustment levers hard to see in the dark so replaced them with some chrome M2.6 standoffs which should show up better.

[markse68]

23 minutes ago, johninderby said:

Couple of mods to the Altair ADC.

Replaced the supplied eyepiece holder with a TS one as the original one took a lot of force to insert an eyepiece. The TS is the same height.

Wanted to be able to rotate the ADC so fitted a short eyepiece holder to the diagonal and shortened a T2 nosepiece in the lathe to suit.

Found the black adjustment levers hard to see in the dark so replaced them with some chrome M2.6 standoffs which should show up better.

have you used it yet then?

[johninderby]

5 minutes ago, markse68 said:

have you used it yet then?

Only once and thought it helped a bit but wanted to ba able to rotate it 180 degrees to see if it was better.

Edited July 30, 2020 by johninderby

[BGazing]

2 hours ago, johninderby said:

Only once and thought it helped a bit but wanted to ba able to rotate it 180 degrees to see if it was better.

You can simply test it against the background...you will find the link in one of my earlier posts to a CN thread where the test and explanation to the test.

[johninderby]

I’ll take a look, thanks.

[BGazing]

Played the whole night with ADC and binos on C8.

The amount of lever correction needed with 1.25GPC is tiny, but results in a detectable improvement. Moons are elongated dispersion dots and when corrected become dancing fuzzballs (seeing, meh) without dispersion. One can also see AD on the Moon this low, while cruising top part it has a sliver of blue fringe (very thin) and the corresponding orange sliver at the southern pole.

Seeing was not that spectacular, tbh, but what can one expect at 23 degrees in meridian.

[astro_al]

A question to ZWO ADC owners please. Step 1 in the manual says "Tighten the centre marker at 'the zero position'". Is the zero position absolute and indicated with an indent in the ADC body or similar?

I was planning on removing the nosepiece and screwing directly to a T2 diagonal, but if the zero position is absolute then I will need to introduce something that allows the whole ADC unit to rotate. If the zero position is relative and the critical part is moving the adjustment levers from zero relative to the horizon, then presumably I can just loosen the centre marker and rotate the ring to the set zero position parallel to the horizon?

Thanks.

Edited July 31, 2020 by astro_al

[markse68]

1 hour ago, BGazing said:

Played the whole night with ADC and binos on C8.

The amount of lever correction needed with 1.25GPC is tiny, but results in a detectable improvement. Moons are elongated dispersion dots and when corrected become dancing fuzzballs (seeing, meh) without dispersion. One can also see AD on the Moon this low, while cruising top part it has a sliver of blue fringe (very thin) and the corresponding orange sliver at the southern pole.

Seeing was not that spectacular, tbh, but what can one expect at 23 degrees in meridian.

Interesting. Seeing wasnt great here either last night- I guess all the heat from a very warm day didn’t help but I also noticed that very little correction was needed on the planets and it didn’t make any noticeable difference in the moon. I wonder if AD effects are inversely proportional to seeing quality? I guess if the ad is swimming around stirred by poor seeing then it’s not a fixed quantity to be fixed with a corrector? 🤔

[CraigT82]

1 hour ago, astro_al said:

A question to ZWO ADC owners please. Step 1 in the manual says "Tighten the centre marker at 'the zero position'". Is the zero position absolute and indicated with an indent in the ADC body or similar?

I was planning on removing the nosepiece and screwing directly to a T2 diagonal, but if the zero position is absolute then I will need to introduce something that allows the whole ADC unit to rotate. If the zero position is relative and the critical part is moving the adjustment levers from zero relative to the horizon, then presumably I can just loosen the centre marker and rotate the ring to the set zero position parallel to the horizon?

Thanks.

The zero position is when the two levers are together and can be anywhere within the zone where the two lever slots overlap. 

[BGazing]

5 hours ago, markse68 said:

Interesting. Seeing wasnt great here either last night- I guess all the heat from a very warm day didn’t help but I also noticed that very little correction was needed on the planets and it didn’t make any noticeable difference in the moon. I wonder if AD effects are inversely proportional to seeing quality? I guess if the ad is swimming around stirred by poor seeing then it’s not a fixed quantity to be fixed with a corrector? 🤔

Yes, you will occasionally see chromatic 'smear', when the chroma is smeared in the east-west direction, or when you correct with ADC and the top blue and bottom orange disappear and then you get chroma occasionally 'dancing around' the edges but as soon as the seeing settles it disappears.

But you can play around with it...return levers to 0 and suddenly there is unmistakable top blue bottom red. Tiny correction and it is gone and everything looks a bit better.

We talk about strehl this spherochromatism that chinese vs premium vs doublet vs triplet but what AD does on planets is that it basically smears those subtle Jove colors all over the place.

 

Edited July 31, 2020 by BGazing

[astro_al]

4 hours ago, CraigT82 said:

The zero position is when the two levers are together and can be anywhere within the zone where the two lever slots overlap. 

Thanks @CraigT82

[fate187]

On 23/07/2020 at 20:08, mikeDnight said:

I can't help it - I have to bite!

Does anyone actually believe that atmospheric cell nonsense anymore?  For it to be at all feasible there would need to be just a single layer of atmospheric cells like a sheet of bubble wrap. In reality the cells overlap and are miles deep, so a small aperture would be just as equally hindered by them as a larger aperture. The argument often claims the larger aperture is hindered because it is looking through more cells. However, if you use two scopes of equal aperture (say 8"), one a reflector and one a refractor, the refractor will almost always give a better planetary image, discrediting the atmospheric cell argument.

Then as regards resolution, a small scope of good quality will resolve detail to its aperture limit. It doesn't present an unresolved image such as a pixelated view of a cooked image. So point source features that are be beyond the resolution limit of a small refractor for example, are easily detectablde if they are linear, despite being too fine to resolve, leaving many observers stunned in disbelief at the laser etched, highly detailed view. 

Hi Mike,

I was reading telescope-optics.net on this topic. You may find the article interesting: https://www.telescope-optics.net/induced.htm#systems 

More specifically certain parameters are defined. According to the model those will align on good seeing conditions for larger aperture to being more effective and smaller during bad or worse conditions. From what I understood this is due to coherence length r0 and turbulence structure constant cn^2. In summary it seems, that the atmosphere can in fact create cells were small diameter optics <=10cm are way less affected than larger scopes. However, the model does have its limitations as the last paragraph states:

The standard Kolmogorov model of turbulence uses certain assumptions which are not strictly met in field conditions. This can significantly affect its accuracy. In general, the actual error is smaller than what the model predicts, as outlined in more details on the next page.

[andrew s]

5 minutes ago, fate187 said:

Hi Mike,

I was reading telescope-optics.net on this topic. You may find the article interesting: https://www.telescope-optics.net/induced.htm#systems 

More specifically certain parameters are defined. According to the model those will align on good seeing conditions for larger aperture to being more effective and smaller during bad or worse conditions. From what I understood this is due to coherence length r0 and turbulence structure constant cn^2. In summary it seems, that the atmosphere can in fact create cells were small diameter optics <=10cm are way less affected than larger scopes. However, the model does have its limitations as the last paragraph states:

The standard Kolmogorov model of turbulence uses certain assumptions which are not strictly met in field conditions. This can significantly affect its accuracy. In general, the actual error is smaller than what the model predicts, as outlined in more details on the next page.

Quite correct, indeed there is no definitive model of turbulence in any setting. An outstanding problem in classical physics.

The Fried number r0 ca be determined experimentally without recourse to any theory of turbulence. 

Regards Andrew 

[Guest]

Having just read all 9 pages of this thread, I think I'll persevere without an ADC for now.  There's other things I'd like to get first, like a RACI finder. 😉

[mikeDnight]

ADC - essential for planetary?  It seems the answer is no!   

That's saved me a few bob! 

 

[JeremyS]

1 hour ago, mikeDnight said:

ADC - essential for planetary?  It seems the answer is no!   

That's saved me a few bob! 

 

Or just get the T-shirt, Mike? 🤔

[mikeDnight]

1 hour ago, JeremyS said:

Or just get the T-shirt, Mike? 🤔

Does that still fit you Jerremy?